1. Field of the Invention
This invention relates to a magnetic recording medium such as a flexible magnetic disk, and more particularly to a magnetic recording medium enabling an optical tracking.
2. Discussion of Related Art
There is known a flexible magnetic disk, for example, from U.S. Pat. No. 4,961,123, in which a reference track is formed at an innermost peripheral portion of a doughnut-like recording region of the disk, and a number of magnetic head-tracking optical recesses of a ring-shape are formed radially outwardly of the reference track at predetermined intervals in concentric relation to the reference track, and the region between any two adjacent rings of magnetic head-tracking optical recesses serves as a data track.
A predetermined signal is beforehand recorded in the above reference track of this magnetic disk. When desired information is to be recorded on this magnetic disk, or when the recorded information is to be reproduced therefrom, the signal in the reference track disposed at the innermost peripheral portion of the recording region are first read by a magnetic head to find a reference position of the magnetic head on the magnetic disk. Then, the magnetic head is shifted a predetermined distance or pitch radially outwardly, and the magnetic head-tracking optical recesses are scanned by optical means, comprising a light-emitting element and a light-receiving element, to effect a tracking servo of the data tracks so as to record or reproduce the information.
FIGS. 18 and 19 are an enlarged plan view and an enlarged cross-sectional view of a part of a conventional reference track, respectively. As shown in FIG. 18, the reference track 100 extends along a direction X of travel of a magnetic head, and a pair of rectangular reference recesses 103A and 103B are provided symmetrically with respect to an arbitrary point 102 on a centerline 101 of the reference track 100. A flat portion 104A with no recess is provided adjacent to the reference recess 103A (that is, forwardly of the reference recess 103B), and also a flat portion 104B with no recess is provided adjacent to the reference recess 103B (that is, rearwardly of the reference recess 103A). A number of pairs of such reference recesses 103A and 103B and a number of pairs of such flat portions 104A and 104B are provided at intervals along the direction X of travel of the magnetic head to thereby form the reference track 100.
As shown in FIG. 19, the reference recesses 103A and 103B are formed by pressing a die against a surface of a magnetic layer 105 to compress it.
Predetermined signals are recorded on the reference track 100 over the entire area thereof, wherein, the recorded signals correspond not to the reference recesses 103A and 103B which are depressed, but to the flat portions 104A and 104B which are not depressed. In accordance with the signal waveform obtained by scanning the reference track 100 by the magnetic head, a feedback control is applied to the present position of the magnetic head, so that the magnetic head can be brought to the centerline 101 (reference position) of the reference track 100.
After the magnetic head is thus located at the reference position on the magnetic disk, the magnetic head is radially outwardly shifted a predetermined distance from this reference position to a desired one of the data tracks.
As shown in FIGS. 18 and 19, each of the conventional reference recesses 103A and 103B is depressed continuously over the entire length thereof, and therefore has a relatively large area. On the other hand, the magnetic layer 105 contains, in addition to magnetic powder, materials such as a binder, an abrasive material, a filler and a lubricant and has a certain degree of elasticity.
Therefore, when the reference recesses 103A and 103B are formed by pressing by means of the die, a central portion 106 of the reference recess 103A (103B) is made to bulge because of a spring back effect due to the elasticity, as indicated by a dot-and-dash line in FIG. 19, so that the recess fails to be a complete recess. This tendency is conspicuous particularly when the length of the reference recess 103A (103B) becomes greater in the direction X of travel of the magnetic head.
Since the reference recesses 103A and 103B are formed by compression, a thin layer 107, which becomes higher in density in the filling of the magnetic powder than the other portions, is formed underneath a bottom surface of the reference recess 103A (103B).
Therefore, when signals are to be recorded on the reference track 100, undesired signals are likely to be recorded also on the central portions of the reference recesses 103A and 103B, and therefore the center of the magnetic head can not be brought properly to the centerline 101 of the reference track 100, which results in a problem with respect to the reliability.